1. Field of the Invention
The present invention relates to a method and apparatus for locating a mobile transceiver operating in its conversation state. More particularly, the invention is directed to causing a mobile transceiver, while in conversation state, to transmit to a plurality of cell sites a predetermined beacon signal as a time difference of arrival reference datum.
2. Description of Related Art
It is desirable that wireless telecommunications service providers be able to geographically locate a mobile wireless transceiver in emergency situations. For example, a kidnap victim might be tracked via his mobile transceiver, including a cellular or PCS telephone. In fact, the United States Federal Communication Commission has mandated under Docket No. CC94-102 that wireless telecommunications service providers be able to locate a mobile transceiver within 125 m with a 67% degree of confidence as of Oct. 1, 2001. This ability is colloquially referred to as enhanced 911 (xe2x80x9cE911xe2x80x9d) service.
E911 service can be implemented from either the mobile transceiver perspective or the network perspective. From the mobile transceiver perspective, one might combine a global positioning system (xe2x80x9cGPSxe2x80x9d) receiver with a wireless mobile transceiver, such that the combination might periodically or on command report its geographic location to a proximate cell site. This solution has a number of disadvantages, including that the mobile transceiver becomes heavy, bulky, power hungry, and complicated compared to a conventional wireless transceiver.
From the network perspective, the problem is approached by realizing that a mobile transceiver in communication with one cell site is generally also detectable by other proximate cell sites. Thus, one can determine the geographic location of the mobile transceiver with reference to the geographic location of each of the cell sites detecting the mobile transceiver and the relative times at which each of these cell sites respectively receives a particular signal from the mobile transceiver. This network approach implements a time difference of arrival (TDOA) calculation.
In estimating at each cell site the time of arrival of a signal from the mobile transceiver, one conventionally detects both the beginning of the mobile transceiver signal and the signal""s phase difference, or intra-symbol time delay. In this regard, it is advantageous that the cell sites know in advance the specific signal to be transmitted by the mobile transceiver.
For example, in a system compliant with interim standard IS-136, this predetermined signal can be obtained from the shortened burst that a mobile transceiver transmits when it initiates a call. This arrangement is satisfactory so long as the mobile transceiver is in a state from which it can initiate a call. For reference, the shortened burst is specified in interim standard IS-136.2.
However, it is also desirable that the mobile transceiver be able to summon help to its location while its user is engaged in an ongoing call, i.e. while the mobile transceiver is in a conversation state. For example, a mobile transceiver user might be speaking with a friend when he detects a need to summon emergency personnel to his location, in which case it might be unsafe or traumatic to terminate the conversation and then dial 911 to determine location.
Unfortunately, current solutions do not support generating a known beacon signal, for example a shortened burst, while a wireless transceiver is in the conversation state.
Aspects of the invention are directed to locating a mobile transceiver in the conversation state. They provide for deliberately forcing the mobile transceiver into a handoff process, wherein without leaving the conversation state, the mobile transceiver transmits a predetermined beacon signal for example a shortened burstxe2x80x94to be received at proximate cell sites. Conventionally, this handoff process is initiated when poor signal quality indicates a need to determine which of the cell sites can best communicate with the mobile transceiver in its current location. However, according to aspects of the invention, this handoff process can also be used to generate a time difference of arrival dataset from the relative times at which the beacon signal respectively arrives at each proximate cell site.
More particularly, according to a preferred embodiment of the invention, the user initiates the locating process by transmitting a flash 911 (xe2x80x9c*911xe2x80x9d) signal from the mobile transceiver to the cell site with which it is currently in communication, requesting that the wireless communication network determine the geographic location of the mobile transceiver.
In response, the cell site receiving the *911 signal transmits to the mobile transceiver a handoff signal, including a synchronization signal. Thereafter, the mobile transceiver synchronizes to the synchronization signal and begins continually transmitting a predetermined beacon signal, for example a shortened burst.
Cell sites sufficiently proximate to the mobile transceiver receive the predetermined beacon signal and with reference to a time standard, ascertain the time of arrival of the beacon signal at that cell site. Each such cell site communicates to a mobile telephone switching office the time at which it received the predetermined beacon signal and, in the event that the mobile telephone switching has not previously stored the cell site""s geographic location, the cell site""s geographic location.
Upon receiving from such cell sites a sufficient dataset of location and time of arrival data, the mobile telephone switching office performs time difference of arrival calculations and thereby resolves the location of the mobile transceiver.
In another embodiment of the invention, another node on the mobile telephone system may initiate the locating process, either with or without the locating process being detectable at the mobile transceiver. In the case of a detectable locating process initiated for example by a friend or a family member, the mobile transceiver may prompt the user before transmitting the beacon signal to determine whether or not the user wants to be located. In the case of an undetectable locating process, advantageous for law enforcement applications, the mobile transceiver would transmit the beacon signal without prompting the user and without leaving the conversation state.
In yet another embodiment of the invention, the mobile transceiver may initiate the locating process for other than emergency purposes. Thus, the mobile telephone network may provide additional services to mobile transceiver users, such that when a user dials a predetermined code or number the mobile transceiver transmits a subscription service signal to the mobile telephone network, which responds by transmitting to the mobile transceiver location data in either visual or audible form.
Thus, more precisely, there is provided according to one aspect of the invention a method including the steps of: transmit ting from one of a plurality of cell sites to a mobile transceiver a handoff signal for causing the mobile transceiver to continually transmit a predetermined beacon signal without leaving a conversation state; at at least some of the plurality of cell sites, receiving the predetermined beacon signal and measuring the time at which the beacon signal was received; and calculating the location of the mobile transceiver from the respective locations of each of the at least some of the plurality of cell sites and the respective times at which the beacon signal was received at each of the at least some of the plurality of cell sites.
The handoff signal might be a signal to the mobile transceiver to handoff to a large diameter cell, might be a Fast Associated Control Channel Handoff (FACCH) signal, and might include a synchronization signal.
It is desirable that the method further include transmitting from one of the plurality of cell sites to the mobile transceiver a cancel signal for causing the mobile transceiver to stop transmitting the predetermined beacon signal and to resume normal conversation.
The cancel signal might include a physical layer control signal, which might in turn include a time alignment signal.
Preferably, calculating the location of the mobile transceiver includes calculating at a mobile telephone switching office the location of the mobile transceiver. In such case, the method might further include receiving at the mobile telephone switching office a query signal and transmitting the handoff signal from one of the plurality of cell sites in response to receiving at the mobile telephone switching office the query signal.
Receiving at the mobile telephone switching office a query signal might advantageously include receiving at the mobile telephone switching office a query signal from the mobile transceiver, including a *911 signal.
According to another aspect of the invention, there is provided a system, including: a mobile telephone switching office having a processor and memory storing codes for instructing the processor to calculate a location of a mobile transceiver based upon the respective locations of a plurality of receivers and the respective times that a beacon signal was received at each of the plurality of receivers; and a plurality of cell sites in communication with the mobile telephone switching office, at least some of the plurality of cell sites having: a transmitter for transmitting to the mobile transceiver a handoff signal for causing the mobile transceiver to continually transmit a predetermined beacon signal without leaving a conversation state; a receiver for receiving from the mobile transceiver the predetermined beacon signal continually transmitted from the mobile transceiver; a clock for measuring the time at which the predetermined beacon signal was received at the receiver; and a communication channel for communicating to the mobile telephone switching office the time at which the predetermined beacon signal was received at the receiver.
According to yet another aspect of the invention, there is provided a method including: receiving at a mobile transceiver from one of a plurality of cell sites a handoff signal for causing the mobile transceiver to continually transmit a predetermined beacon signal without leaving a conversation state; and transmitting to at least some of the plurality of cell sites, the predetermined beacon signal for time difference of arrival calculation.
According to still another aspect of the invention, there is provided a cellular mobile transceiver apparatus locatable in a conversation state with respect to a plurality of cell sites, having: a receiver for receiving from at least one of the plurality of cell sites a handoff signal for causing the mobile transceiver to continually transmit a predetermined beacon signal while still in the conversation state; and a transmitter for transmitting the predetermined beacon signal to at least some of the plurality of cell sites for time difference of arrival calculation in response to the handoff signal being received at the receiver.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.